The invention relates to an anchor line pretensioning method comprising the steps of:                connecting an anchor line with a first end and a tensioning line with a first end to the seabed, at opposite sides of an installation vessel,        connecting the anchor line and the tensioning line with their second ends to the vessel, at least one line being connected to a tensioning device on the vessel, and        pulling at least one line in its length direction towards the vessel.        
Mooring systems for offshore structures, such as Floating Production Storage and Offloading Systems (FPSO's), spar buoys, barges and the like, usually comprise arrays of catenary anchor lines. These anchor lines extend along a curved trajectory from the floating structure, to the seabed. The anchor lines may be connected to the seabed by fluke anchors, suction anchors, driven piles or suction piles. In case drag anchors are used, it is necessary to apply a pretension load on the anchor lines after installation to bury the anchors in the seabed and to have the drag anchors develop their design holding power. In case other types of anchoring are used with deep attachment points, which may be located 5–10 meters below the surface of the seabed, the pretension load on the anchor lines will allow the lines to cut through the soil near the anchor pint such that future slackening of the anchor lines is prevented. Furthermore, in each case the pretension load on the anchor line ensures that the line is laid in a straight line along its required route for anchoring the offshore structure.
One way of tensioning anchor lines is to apply a load by the propulsion of one or more tugs. Another method is by pulling the anchor line against the anchor lines of the installation vessel by which the anchor lines are put in place. These methods may impose severe requirements on the installation spread and are relatively costly.
In case the anchor lines are arranged in an array comprising two anchor lines connected to diametrically opposed anchor points located on each side of the floating structure, the anchor lines can be pulled against each other on the installation vessel using either a pulling device across the deck, a crane or a subsea tensioner which requires a relatively cheap installation spread.